This invention relates to current mirroring circuits and the use thereof in a plurality of integrated circuit applications and more particularly, to a plurality of inverted transistor current mirroring and pull down circuits for such applications.
The prior art is replete with different types of integrated circuits having current mirroring circuits. For example, current mirroring circuits are utilized in differential amplifier circuits to reduce input voltage offset errors at the quiescent operating state of such amplifiers. Such current mirroring circuits are also utilized in gaining and/or level shifting any differential or single-ended signal.
Presently, some prior art current mirroring circuits utilize a diode, more commonly, a diode-connected transistor connected in parallel with the base-emitter junction of a transistor. The diode and transistor are then connected in the collector circuitry of the differential amplifier as is understood. Equal quiescent currents are established by making the PN junctions of the diode and transistor equal in area as is known. An additional, but less accurate, method for providing current mirroring is to utilize a base pull down resistor in place of the diode. Both of these circuits are illustrated hereinafter.
In the design of large scale integrated (LSI) circuits chip space is often limited. Therefore, a need exists for reducing the number of components and devices required to perform standard circuit functions, e.g., current mirroring. A disadvantage of the prior art current mirroring circuits is that at least two component devices are required to provide this function, each requiring a minimum area on the chip. Thus a need exists for reducing component count required for providing the current mirroring function to subsequently reduce required chip area.
In some comparator circuit applications it is desirous to ensure that an output signal is generated only when the input signal is of a particular polarity and to positively inhibit any output signal if the input signal is of the opposite polarity. In these prior art circuits, current mirroring is utilized to drive multiple devices to actively pull down the output of the comparator to essentially ground reference to inhibit any output signal. Again, however, the use of multiple component devices is required to provide this function and these techniques suffer from the same disadvantages discussed above.
The present invention overcomes the problems of the above circuits by utilizing inverted transistor current mirroring and pull down circuits to minimize component count. The use of a single inverted transistor with multiple emitters to provide the above functions significantly reduces to a fraction the area required on the chip by multiple transistors. Inverted transistors as used in applications herein are typical vertical bipolar transistors wherein the emitter electrodes are utilized as collectors with the collector electrodes functioning as emitters as is known in the art.